The present invention relates to an output compensating device for an image sensor and, more specifically, to an output compensating device for compensating for variations in pixel outputs of a MOS type image sensor.
Japanese Laid-Open Patent Publication No. 2000-329616 discloses a conventional MOS transistor type image sensor, in which a light sensor circuit used as a unit pixel comprises, as shown in FIG. 1, a photo-diode PD operating as a photoelectric converting element for producing a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the sensor current produced in the photodiode into a voltage signal Vpd, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting a sensor signal in accordance with a timing pulse of a readout signal Vs and which circuit is characterized by its a wide dynamic range obtained by giving the output a logarithmic characteristic, thereby achieving the high sensitivity of detecting a light signal. In addition, the light sensor circuit is provided with a means for changing a drain voltage VD of the transistor Q1 to a value lower than a normal value for a specified period to remove a charge accumulated in a parasitic capacitor C of the photodiode PD to initialize the circuit. The light sensor circuit can thus obtain a voltage signal Vpd corresponding to the quantity of incident light Ls even if the sensor current absurdly changed, thereby eliminating the possibility of occurrence of afterglow of each pixel even at a small quantity of incident light.
As shown in FIG. 3, the above-described light sensor circuit may output a signal with a logarithmic characteristic at a normal quantity of sensor current corresponding to a normal quantity of incident light to a photodiode but it may not maintain the logarithmic output characteristic and may have a substantially linear output characteristic at a decreased sensor current due to a delay of charging the parasite capacitor of the photodiode. In FIG. 3, WA represents a region of responding with a non-logarithmic characteristic output and WB represents a region of responding with a logarithmic characteristic output.
However, the conventional image sensor using the light sensor circuits, in which a sensor current proportional to incident light is produced in a photo-electric converting element and converted into a voltage signal by using a MOS type transistor having logarithmic output characteristic in a weak inverse state, still involves such a problem that it cannot be free from the occurrence of unwanted afterglow of each pixel with a decreased quantity of incident illumination falling on the photoelectric converting element.
The MOS type image sensor may suffer variations in output characteristics of pixel signals, which may be resulted from structure-derived variations in output characteristics and variations in temperature characteristics of respective light sensor circuits.
Japanese Laid-Open Patent Publication No. 2000-175108 discloses a conventional image sensor using the light sensor circuits as respective pixels, in which a sensor current proportional to incident light is produced in a photo-electric converting element and converted into a voltage signal by using a MOS type transistor having logarithmic output characteristic in a weak inverse state, wherein variations in its output with a change in temperature (as shown in FIG. 37) are compensated for according to the image sensor temperature detected by a temperature sensor.
FIG. 41 shows a construction of a conventional image sensor, in which a sensor signal AlSig from the image sensor 7 is compensated for temperature variations by doing digital operations according to a temperature signal Atemp detected by a temperature sensor 12.
In the image sensor of FIG. 41, each of sensor signals AlSig output from the image sensor 7 is converted by an A-D converter IADC into a digital signal DlSig that is then transferred to a digital temperature compensation circuit DCAL. A temperature signal Atemp from a temperature sensor 12 provided in the image sensor 7 is also converted by an A-D converter TADC into a digital signal Dtemp that is then transferred to a lookup table LUT of characteristic offset compensation values preset versus variable temperatures. An offset compensation value DtempOFS corresponding to the currently detected temperature of the image sensor 7 is selected from the lookup table LUT and given to the digital temperature compensation circuit DCAL by which the digital sensor signal DlSig is corrected by adding to or subtracting from the signal the given offset compensation value DtempOFS. The compensated digital sensor signal can thus be obtained. When compensating sensor signals from the image sensor are processed by the digital compensation circuit, each of pixel sensor signals and a temperature detection signal from the temperature sensor are necessarily converted by A-D converters into digital signals. However, if sensor signals might vary in a wide range as the temperature changes, the A-D converter for converting the sensor signals must be set to a wide input range. This limits the use of the A-D converter. Furthermore, the temperature compensation of sensor signals by calculations may require the provision of a complicated compensating configuration.